Beam homogenizer, laser irradiation apparatus, and method for manufacturing semiconductor device

ABSTRACT

The energy distribution of the beam spot on the irradiated surface changes due to the change in the oscillation condition of the laser or before and after the maintenance. The present invention provides an optical system for forming a rectangular beam spot on an irradiated surface including a beam homogenizer for homogenizing the energy distribution of the rectangular beam spot on the irradiated surface in a direction of its long or short side. The beam homogenizer includes an optical element having a pair of reflection planes provided oppositely for reflecting the laser beam in the direction where the energy distribution is homogenized and having a curved shape in its entrance surface. The entrance surface of the optical element means a surface of the optical element where the laser beam is incident first.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a beam homogenizer to homogenize energydistribution of a beam spot on an irradiated surface in a particularregion. Moreover, the present invention also relates to a laserirradiation apparatus using the beam homogenizer. Furthermore, thepresent invention relates to a method for manufacturing a semiconductordevice using the laser irradiation apparatus.

2. Related Art

In recent years, a technique has been extensively researched in whichthe laser annealing is performed to a non-single crystal semiconductorfilm (the non-single crystal semiconductor includes an amorphoussemiconductor and a semiconductor having crystallinity such aspoly-crystal or micro-crystal, which is not single crystal) formed overan insulating substrate such as a glass substrate. It is noted that thelaser annealing described herein indicates a technique to recrystallizean amorphous layer or a damaged layer formed in a semiconductorsubstrate or in the semiconductor film and a technique to crystallizethe non-single crystal semiconductor film formed over the substrate.Moreover, a technique applied to planarize or modify a surface of thesemiconductor substrate or the semiconductor film is included in thelaser annealing.

The laser annealing is performed in the crystallization because theglass substrate has a low melting point. The laser can give high energyonly to the non-single crystal semiconductor film without changing thetemperature of the substrate that much.

It is preferable to perform the laser annealing in such way that apulsed laser beam having high output such as an excimer laser is shapedinto a square spot having a length of several cm on a side or into arectangular spot having a length of 10 cm or more on a longer side at anirradiated surface through an optical system and that an irradiationposition of the beam spot is scanned relative to the irradiated surfacebecause this method can enhance productivity and is superiorindustrially. Among the rectangular beam spots, a rectangular beam spothaving a high aspect ratio is referred to as a linear beam spot in thisspecification.

In particular, unlike a punctuate beam spot requiring to be scanned fromfront to back and from side to side, the linear beam spot can providehigh productivity because the linear beam spot can be irradiated to thelarge irradiated surface by scanning the linear beam spot only in adirection perpendicular to the long-side direction of the linear beamspot. The laser beam is scanned in the direction perpendicular to thelong-side direction of the linear beam spot because it is the mosteffective way to scan the laser beam. Because of such high productivity,at present, the laser annealing process mainly employs the linear beamspot obtained by shaping the beam spot emitted from a pulsed excimerlaser through an appropriate optical system.

FIGS. 12A and 12B show an example of an optical system to transform thesectional shape of the beam spot into linear on the irradiated surface.The optical system shown in FIGS. 12A and 12B is an extremely generaloptical system. This optical system not only transforms the sectionalshape of the beam spot into linear, but also homogenizes the energydistribution of the beam spot on the irradiated surface at the sametime. Generally, the optical system for homogenizing the energydistribution of the beam spot is referred to as a beam homogenizer. Theoptical system shown in FIGS. 12A and 12B is also the beam homogenizer.

When a XeCl excimer laser (having a wavelength 308 nm) is used as alight source, the above optical system is made of quartz. When a laserhaving a shorter wavelength is used as the light source, the opticalsystem is made of fluorite, MgF₂, or the like.

First, a side view of FIG. 12A is explained. A laser beam emitted from aXeCl excimer laser oscillator 1201 is divided in one direction throughcylindrical lens arrays 1202 a and 1202 b. This direction is hereinreferred to as a vertical direction. When a mirror is inserted in theoptical system, the vertical direction is bent to the direction of thelaser beam bent by the mirror. In this structure, the laser beam isdivided into four beams. These divided beams are combined into one beamspot once by a cylindrical lens 1204. The beam spots separated again arereflected by a mirror 1206 and then are condensed into one beam spotagain on an irradiated surface 1208 by a doublet cylindrical lens 1207.The doublet cylindrical lens is a lens consisting of two cylindricallenses. This homogenizes the energy distribution of the linear beam spotin the vertical direction and determines the length thereof in thevertical direction.

Next, a top view of FIG. 12B is explained. The laser beam emitted fromthe laser oscillator 1201 is divided in a direction perpendicular to thevertical direction by a cylindrical lens array 1203. The directionperpendicular to the vertical direction is herein referred to as ahorizontal direction. When a mirror is inserted in the optical system,the horizontal direction is bent to the direction of the beam bent bythe mirror. In this structure, the laser beam is divided into sevenbeams. These divided beams are combined into one beam spot by acylindrical lens 1205 on the irradiated surface 1208. A dotted lineshows a correct optical path and correct positions of the lens and theirradiated surface in the case not disposing the mirror 1206. Thishomogenizes the energy distribution of the linear beam spot in thehorizontal direction and determines the length thereof in the horizontaldirection.

As described above, the cylindrical lens arrays 1202 a, 1202 b, and 1203are the lenses to divide the beam spot of the laser beam. Thehomogeneity of the energy distribution of the obtained linear beam spotdepends on the number of the divided beam spots.

In general, the excimer laser emits a rectangular laser beam having anaspect ratio in the range of approximately 1 to 5. The beam spot of thelaser beam has Gaussian distribution where the intensity is highertoward the center. The optical system shown in FIGS. 12A and 12Btransforms the beam spot so as to form the beam spot having homogeneousenergy distribution and having a size of 320 mm×0.4 mm

The linear beam spot shaped by the above structure is irradiated asbeing overlapped in such a way that the linear beam spot is displacedgradually in the direction of the short side of the linear beam spot.Such an irradiation method makes it possible to perform the laser,annealing to the whole surface of the non-single crystal silicon film tocrystallize it or to enhance its crystallinity. In a mass-productionfactory, at present, the laser annealing is performed to thesemiconductor film using the linear beam spot shaped by the opticalsystem as above. Some beam homogenizers use a reflection mirror. (Forexample, patent document 1)

[Patent Document 1] Japanese Patent Unexamined Publication No.2001-291681 bulletin

However, a laser irradiation apparatus using the pulsed excimer laserhas a problem that, for example, the homogeneity of the energydistribution of the beam spot on the irradiated surface deterioratesbecause of the fluctuation of a beam axis, which is explained later, orthe change in the divergence angle of a laser beam due to the change inthe oscillation condition of the excimer laser or due to the cleaning ofthe window for isolating the gas, which is the laser medium of theexcimer laser, from the outside. Therefore, such a laser irradiationapparatus is not yet of high quality for the mass production. The term“beam axis” herein used means a path in which the laser beam travels.The fluctuation or the change of the beam axis means that of the traveldirection of the laser beam including the parallel shift of the traveldirection of the laser beam.

SUMMARY OF THE INVENTION

The present invention is made in view of the above problem, and it is anobject of the present invention to provide a beam homogenizer that cansuppress as much as possible the change in the energy distribution ofthe beam spot on the irradiated surface due to the change in theoscillation condition of the excimer laser or due to the maintenance.Moreover, it is an object of the present invention to provide a laserirradiation apparatus and a method for manufacturing a semiconductordevice that use the beam homogenizer.

In the present invention, an optical element having a pair of reflectionplanes provided oppositely and having a curved shape in its entrancesurface where the laser beam is incident is used to homogenize theenergy distribution of the beam spot on the irradiated surface. As suchan optical element, there are a light pipe and an optical waveguide forexample. The light pipe is an optical element made of a transparentmaterial having a shape of rectangular solid, circular conic, pyramid,cylinder, or the like, which transmits the light from one end to theother end by the total reflection. The optical waveguide is an opticalelement that can confine radiation light to a certain region and cantransmit the radiation light by guiding the flow of the beam thereof. Itis noted that reflection by a mirror may be used to transmit the light.The light pipe belongs to a field of the illumination optics while theoptical waveguide belongs to a field of optical communication, which istypified by an optical fiber. Although these two optical elements belongto the different field, it can be said that their optical actions arealmost the same.

The present invention discloses a beam homogenizer for shaping a beamspot on an irradiated surface into rectangular. The beam homogenizerincludes an optical element for homogenizing energy distribution of therectangular beam spot in a direction of its long or short side on theirradiated surface, wherein the optical element has a curved shape inthe entrance surface where the laser beam is incident and wherein theoptical element has a pair of reflection planes provided oppositely. Theentrance surface herein means a surface of the optical element where thelaser beam is incident first. As the curved shape, a lens is given forexample.

In the present invention, the optical element has the curved shape inits entrance surface where the laser beam is incident according to thefollowing reason. When the center axis of the beam axis does not matchthe center axis of the optical element, the laser beam is incidentobliquely into the entrance surface of the optical element. When thelaser beam is incident obliquely into the entrance surface of theoptical element, the reflection of the laser beam in the optical elementis asymmetrical to the center axis of the optical element. Therefore,the energy distribution of the beam spot at the exit surface of theoptical element is not sufficiently homogenized. Consequently, thepresent invention provides the optical element having the curved shapein its entrance surface. With this optical element, the reflection ofthe laser beam in the optical element can be made symmetrical or nearsymmetrical to the center axis of the optical element. This canhomogenize the energy distribution of the beam spot at the exit of theoptical element.

The present invention discloses another beam homogenizer for shaping abeam spot on an irradiated surface into rectangular. This beamhomogenizer includes an optical element for homogenizing energydistribution of the rectangular beam spot in a direction of its long orshort side on the irradiated surface and includes one or a plurality ofcylindrical lenses for projecting a plane having homogeneous energydistribution formed by the optical element to the irradiated surface,wherein the optical element has a curved shape in its entrance surfacewhere the laser beam is incident and wherein the optical element has apair of reflection planes provided oppositely.

The present invention discloses another beam homogenizer for shaping abeam spot on an irradiated surface into rectangular. This beamhomogenizer includes a plurality of optical elements including at leasta first optical element for homogenizing energy distribution of therectangular beam spot in a direction of its long side on the irradiatedsurface and a second optical element for homogenizing energydistribution of the rectangular beam spot in a direction of its shortside on the irradiated surface, wherein each of the first and secondoptical elements has a curved shape in its entrance surface where thelaser beam is incident and wherein the first and second optical elementsrespectively have a pair of reflection planes provided oppositely.

In the beam homogenizer disclosed in the present invention for shaping abeam spot on an irradiated surface into rectangular, a light pipe or anoptical waveguide can be used as the optical element for homogenizingthe energy distribution of the rectangular beam spot on the irradiatedsurface in the direction of its long or short side.

In the beam homogenizer of the present invention for shaping a beam spoton an irradiated surface into rectangular, the curved shape iscylindrical and has curvature in a direction where the optical elementacts.

In the present invention, the beam homogenizer shapes a beam spot on theirradiated surface into a rectangular beam spot having an aspect ratioof 10 or more, preferably 100 or more.

The present invention discloses a laser irradiation apparatus forshaping a beam spot on an irradiated surface into rectangular. Thislaser irradiation apparatus includes a laser oscillator and a beamhomogenizer wherein the beam homogenizer includes an optical element forhomogenizing energy distribution of the rectangular beam spot in adirection of its long or short side, wherein the optical element has acurved shape in its entrance surface where the laser beam is incidentand wherein the optical element has a pair of reflection planes providedoppositely.

The present invention discloses another laser irradiation apparatus forshaping a beam spot on an irradiated surface into rectangular. Thislaser irradiation apparatus includes a laser oscillator, a beamhomogenizer, and one or a plurality of. cylindrical lenses forprojecting a plane having homogeneous energy distribution formed by thebeam homogenizer, wherein the beam homogenizer includes an opticalelement for homogenizing energy distribution of the rectangular beamspot in a direction of its long or short side, wherein the opticalelement has a curved shape in its entrance surface where the laser beamis incident, and wherein the optical element has a pair of reflectionplanes provided oppositely.

The present invention discloses another laser irradiation apparatus forshaping a beam spot on an irradiated surface into rectangular. Thislaser irradiation apparatus includes a laser oscillator and a beamhomogenizer wherein the beam homogenizer includes a plurality of opticalelements including at least a first optical element for homogenizing theenergy distribution of the rectangular beam spot in a direction of itslong side and a second optical element for homogenizing the energydistribution thereof in a direction of its short side, wherein each ofthe first and the second optical elements has a curved shape in itsentrance surface where the laser beam is incident, and wherein the firstand second optical elements respectively have a pair of reflectionplanes provided oppositely.

In the laser irradiation apparatus disclosed in the present invention, alight pipe or an optical waveguide can be used as the optical elementfor homogenizing the energy distribution of the rectangular beam spot onthe irradiated surface in the direction of its long or short side.

In the above laser irradiation apparatus of the present invention, thecurved shape is cylindrical shape and has the curvature in a directionwhere the optical element acts.

The laser irradiation apparatus of the present invention shapes a beamspot on the irradiated surface into a rectangular beam spot having anaspect ratio of 10 or more, preferably 100 or more.

The laser irradiation apparatus of the present invention has a scanningstage for moving an irradiated object having an irradiated surfacerelative to a beam spot and has an automatic transferring apparatus fortransferring the irradiated object having the irradiated surface to thescanning stage.

In the laser irradiation apparatus of the present invention, the laseroscillator is one selected from the group consisting of an excimerlaser, a YAG laser, a glass laser, a YVO₄ laser, a YLF laser, an Arlaser, and a GdVO₄ laser.

The present invention discloses a method for manufacturing asemiconductor device including the steps of forming a non-single crystalsemiconductor film over a substrate and performing laser annealing insuch a way that a laser beam which is emitted from a laser oscillatorand which is shaped into a rectangular beam spot on the non-singlecrystal semiconductor film through an optical system including anoptical element for homogenizing energy distribution of the rectangularbeam spot is irradiated to the non-single crystal semiconductor whilemoving a position of the beam spot, wherein the optical element acts ona direction of a long or short side of the rectangular beam spot,wherein the optical element has a curved shape in its entrance surfacewhere the laser beam is incident, and wherein the optical element has apair of reflection planes provide oppositely.

The present invention discloses another method for manufacturing asemiconductor device comprising the steps of forming a non-singlecrystal semiconductor film over a substrate and performing laserannealing in such a way that a laser beam which is emitted from a laseroscillator and which is shaped into a rectangular beam spot on thenon-single crystal semiconductor film through an optical systemincluding an optical element for homogenizing energy distribution of arectangular beam spot and one or a plurality of cylindrical lenses forprojecting a plane having the homogeneous energy distribution formed bythe optical element to the non-single crystal semiconductor film isirradiated to the non-single crystal semiconductor film while moving aposition of the beam spot wherein optical element acts on a direction ofa long or short side of the rectangular beam spot, wherein the opticalelement has a curved shape in its entrance surface where the laser beamis incident, and wherein the optical element has a pair of reflectionplanes provided oppositely.

The present invention discloses another method for manufacturing asemiconductor device including the steps of forming a non-single crystalsemiconductor film over a substrate and performing laser annealing insuch a way that a laser beam which is emitted from a laser oscillatorand which is shaped into a rectangular beam spot on the non-singlecrystal semiconductor film through an optical system including aplurality of optical elements is irradiated to the non-single crystalsemiconductor film while moving a position of the beam spot wherein theplurality of optical elements includes at least a first optical elementacting on a direction of its long side of the rectangular beam spot anda second optical element acting on a direction of its short side of therectangular beam spot, wherein each of the first and second opticalelements has a curved shape in its entrance surface where the laser beamis incident, and wherein the first and second optical elementsrespectively have a pair of reflection planes provided oppositely.

In the method for manufacturing a semiconductor device of the presentinvention, a light pipe or an optical waveguide can be used instead ofan optical element to homogenize the energy distribution of therectangular beam spot on the irradiated surface in a direction of itsshort side in the optical system for forming a rectangular beam spot.

In the method for manufacturing a semiconductor device of the presentinvention, the curved shape is cylindrical and has the curvature in adirection where the optical element acts.

In the method for manufacturing a semiconductor device of the presentinvention, the rectangular beam spot formed on the irradiated surfacehas an aspect ratio of 10 or more, preferably 100 or more.

In the method for manufacturing a semiconductor device of the presentinvention, the laser oscillator is one selected from the groupconsisting of an excimer laser, a YAG laser, a glass laser, a YVO₄laser, a YLF laser, an Ar laser, and a GdVO₄ laser.

ADVANTAGEOUS EFFECT OF THE INVENTION

When the beam homogenizer including the optical element that homogenizesthe energy distribution of the laser beam and that has a curved shape inits entrance surface where the laser beam is incident for forming arectangular beam spot disclosed in the present invention is used, it ispossible to form a rectangular beam spot having homogeneous energydistribution on the irradiated surface. Moreover, since the position andthe energy distribution of the beam spot formed on the irradiatedsurface are not easily affected by the oscillation condition of thelaser oscillator, it is possible to keep the shape of the beam spotstably.

When the rectangular beam spot emitted from the laser irradiationapparatus using the beam homogenizer of the present invention is scannedon a semiconductor film in a direction of its short side, it is possibleto suppress the inhomogeneous crystallinity due to the inhomogeneousenergy distribution of the beam spot and to improve the homogeneity ofthe crystallinity in the surface of the substrate. Moreover, accordingto the present invention, the laser irradiation apparatus can obtain thehigh stability. Furthermore, since it is possible to do the maintenancemore easily, the running cost can be reduced. With the present inventionapplied to the mass production line of the poly-silicon TFT, a TFThaving high operating characteristic uniformly can be manufacturedefficiently. Moreover, when the poly-silicon TFT manufactured by thepresent invention is applied to a liquid crystal display device and alight-emitting device using a light-emitting element typified by an ELelement, it is possible to manufacture a display device having almost nodisplay unevenness.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a drawing for explaining the beam homogenizer of the presentinvention;

FIG. 2 is a drawing for explaining the conventional beam homogenizer;

FIGS. 3A and 3B are drawings for showing an example of the laserirradiation apparatus including the beam homogenizer disclosed in thepresent invention;

FIGS. 4A and 4B are drawings for showing an example of the laserirradiation, apparatus including the beam homogenizer disclosed in thepresent invention;

FIGS. 5A to. 5C are drawings for showing the energy distribution of thebeam spot;

FIGS. 6A and 6B are drawings for showing an example of the laserirradiation apparatus including the beam homogenizer disclosed in thepresent invention;

FIGS. 7A and 7B are drawings for showing an example of the laserirradiation apparatus including the beam homogenizer disclosed in thepresent invention;

FIGS. 8A and 8B are drawings for showing an example of the laserirradiation apparatus including the beam homogenizer disclosed in thepresent invention;

FIG. 9 is a drawing for showing the energy distribution of the beamspot;

FIGS. 10A and 10B are drawings for showing an example of the laserirradiation apparatus including the beam homogenizer disclosed in thepresent invention;

FIGS. 11A and 11B are drawings for explaining the homogenization of theenergy distribution by the optical waveguide;

FIGS. 12A and 12B are drawings for explaining the related art;

FIGS. 13A and 13B are drawings for showing the energy distribution ofthe beam spot;

FIGS. 14A and 14B are drawings for showing the energy distribution ofthe beam spot;

FIGS. 15A and 15B are drawings for showing the energy distribution ofthe beam spot;

FIG. 16 is a drawing for explaining the incidence angle of the laserbeam; and

FIGS. 17A and 17B are drawings for showing an example of the laserirradiation apparatus including the beam homogenizer disclosed in thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, an embodiment mode and embodiments of the present inventionare explained based on drawings. However, since the present inventioncan be embodied in many different modes, it is easily understood bythose skilled in the art that the modes and the details of the presentinvention can be changed and modified in various ways unless suchchanges and modifications depart from the scope and the content of thepresent invention hereinafter defined. Thus, the present invention isnot limited to the description of the embodiment mode and theembodiments. It is noted that the same reference numeral is used toindicate the same thing throughout the drawings of the presentinvention.

First, the method to homogenize the energy distribution of the beam spotby the optical element having a pair of reflection planes providedoppositely for reflecting the laser beam in the direction where theenergy distribution of the beam spot is homogenized is explained withreference to FIGS. 11A and 11B. A top view of FIG. 11A is explainedfirst. There are an optical element 1102 having a pair of reflectionplanes 1102 a and 1102 b provided oppositely and an irradiated surface1103 in FIG. 11A. The ray is made incident from the left side on thepaper. The ray is drawn with a continuous line 1101 a when there is theoptical element 1102 and is drawn with a dotted line 1101 b when thereis not the optical element 1102. When there is not the optical element1102, the ray incident from the left side on the paper reaches regions1103 a, 1103 b, and 1103 c in the irradiated surface 1103 as shown withthe dotted line 1101 b.

On the other hand, when there is the optical element 1102, the ray isreflected by the reflection planes of the optical element 1102 as shownwith the ray 1101 a, and then all the rays reach a region 1103 b in theirradiated surface 1103. That is to say, when there is the opticalelement 1102, all the rays that reach the regions 1103 a and 1103 c whenthere is not the optical element 1102 reach the region 1103 b in theirradiated surface 1103. Therefore, when the rays are made incident intothe optical element 1102, the rays are reflected repeatedly and are ledto the exit. That is to say, the rays are superposed in the sameposition as if the incident rays are folded on the region 1103 b in theirradiated surface 1103. In this example, the total divergence of therays 1103 a, 1103 b, and 1103 c on the irradiated surface 1103 whenthere is not the optical element is defined as A, and the divergence ofthe ray 1103 b on the irradiated surface 1103 when there is the opticalelement is defined as B. Then, A/B corresponds to the number of raysdivided by the homogenizer. Thus, when the incident ray is divided andall the divided rays are superposed on the same position, the energydistribution of the ray is homogenized on the superposed position.

In general, the more the homogenizer divides the ray, the morehomogeneous the energy distribution becomes on the position where thedivided rays are superposed. The number of divisions by the opticalelement 1102 can be increased when the ray is reflected more times inthe optical element 1102. In other words, the length of the pair ofreflection planes in the direction where the rays are incident may bemade longer. Moreover, the number of divisions can be increased bynarrowing the space between the reflection planes provided oppositely orby increasing NA (numerical aperture) of the ray incident into theoptical element.

The light pipe or the optical waveguide, having a pair of reflectionplanes provided oppositely for reflecting the laser beam in thedirection where the energy distribution of the beam spot is homogenized,can be used as the optical element to homogenize the energy distributionof the ray.

The optical system for forming a rectangular beam spot disclosed in thepresent invention is explained with reference to FIGS. 3A and 3B. First,a side view of FIG. 3B is explained. A laser beam emitted from a laseroscillator 301 propagates in a direction indicated by an arrow in FIGS.3A and 3B. The laser beam is expanded by spherical lenses 302 a and 302b. The spherical lenses 302 a and 302 b are not required in the casewhere the beam spot emitted from the laser oscillator 301 issufficiently large.

The direction of the long side hereinafter means the direction of thelong side of the rectangular beam spot formed on the irradiated surface307. The direction of the short side hereinafter means the direction ofthe short side of the rectangular beam spot formed on the irradiatedsurface 307. The laser beam is focused by a cylindrical lens 304 in adirection of the short side and is incident into an optical element 305positioned behind the cylindrical lens 304, having a pair of reflectionplanes provided oppositely for reflecting the laser beam in thedirection where the energy distribution of the beam spot is homogenized.The entrance surface of the optical element 305 has cylindricalcurvature in a direction of the short side, which means the directionwhere the energy distribution is homogenized, The laser beam is totallyreflected in the light pipe repeatedly and is led to the exit. Then, aplane having homogeneous energy distribution in a direction of the shortside of the rectangular beam spot is formed at an exit surface of theoptical element 305. Here, the exit surface means a surface of theoptical element from which the laser beam is emitted. It is necessary todetermine the curvature of the cylindrical lens 304 so that the laserbeam is totally reflected at the interface between the optical element305 and the air.

The reason why the optical element 305 has the curvature in the entrancesurface thereof is explained with reference to FIGS. 1 and 2. FIGS. 1and 2 are the drawings observed from a direction perpendicular to thedirection where the energy distribution is homogenized. In FIGS. 1 and2, a laser beam emitted from a laser oscillator (not showndiagrammatically) is focused by cylindrical lenses 101 and 201 so thatthe laser beam is incident into optical elements 102 and 202. Theincidence position of the laser beam is not the center of thecylindrical lenses 101 and 201 in both FIGS. 1 and 2, and the focalpoints are not on the center axis of the optical elements 102 and 202,having a pair of reflection planes provided oppositely for reflectingthe laser beam in the direction where the energy distribution of thebeam spot is homogenized. The laser beams are incident obliquely at acertain angle into the optical elements 102 and 202.

In FIG. 2, the laser beam is focused by the cylindrical lens 201 and isincident into the optical element 202 obliquely. After that, the laserbeam is reflected repeatedly in the optical element asymmetrically tothe center axis of the light pipe and is led to the exit. Thus, a beamspot having inhomogeneous energy distribution is formed at the exitsurface of the optical element. On the other band, in FIG. 1, after thelaser beam is focused by the cylindrical lens 101, the laser beam isincident into the optical element 102 having the curved shape in theentrance surface thereof. When the optical element has the curved shapein the entrance surface, the laser beam incident obliquely into theoptical element expanded to correct the reflection of the laser beamincident into the optical element so that the reflection becomessymmetrical or near symmetrical to the center axis of the opticalelement. This can form the beam spot having homogeneous energydistribution at the exit surface of the optical element.

The curvature of the curved shape is determined based on thespecification of the optical system in the previous paragraph such asthe incidence angle and the length and the width of the optical element.

As described above, when the optical element having the curved shape inits entrance surface is used, the energy distribution of the beam spotformed by the optical element can be kept homogeneous even though thebeam axis changes for every pulse oscillation or even though the beamaxis changes due to the maintenance or due to the effect of the pointingstability of the laser beam emitted from the laser oscillator. Moreover,with the optical element, the position of the plane having homogeneousenergy distribution can be fixed completely by the optical system. Thismakes it possible to obtain the laser beam having homogeneous energydistribution that is not affected by the condition of the laseroscillator on the irradiated surface.

The longer the optical element 305 is in the direction where the laserbeam is incident or the shorter the focal length of the cylindrical lens304 is, the more homogeneous the energy distribution becomes. However,since the actual system must be manufactured in consideration of thesize of the optical system, it is necessary that the length of the lightpipe and the focal length are practical in accordance with the size ofthe system.

In FIGS. 3A and 3B, a doublet cylindrical lens 306 positioned behind theoptical element 305 projects the plane having homogeneous energydistribution formed at the exit surface of the optical element 305 to anirradiated surface 307 positioned behind the doublet cylindrical lens.The doublet cylindrical lens 306 is a lens consisting of two cylindricallenses 306 a and 306 b. This projects the plane having homogeneousenergy distribution formed at the exit surface of the optical element305 to another surface (irradiated surface). In other words, the planehaving homogeneous energy distribution and the irradiated surface 307are in the conjugated position with respect to the doublet cylindricallens 306. The optical element 305 and the doublet cylindrical lens 306homogenize the energy distribution of the rectangular beam spot in thedirection of its short side and determine the length thereof in thedirection of its short side. In addition, when the homogeneity of theenergy distribution of the beam spot at the irradiated surface is notrequired that much or when the F-number (F=focal length of thelens/diameter of the entrance pupil) of the doublet cylindrical lens isextremely large, a singlet cylindrical lens may be used.

Next, a top view of FIG. 3A is explained. A beam spot of a laser beamemitted from a laser oscillator 301 is divided by a cylindrical lensarray 303 in a direction of a long side. The cylindrical lens array 303has a plurality of cylindrical lenses arranged in a direction of itscurvature. In the present embodiment mode, five cylindrical lenses arearranged in the cylindrical lens array. This homogenizes the energydistribution of the rectangular beam spot in a direction of its longside and determines the length thereof in the direction of its longside. It is noted that a cylindrical lens for combining the laser beamsdivided by the cylindrical lens array may be provided behind thecylindrical lens array.

The laser oscillator used in combination with the beam homogenizer ofthe present invention preferably has high output power and has awavelength region that is sufficiently absorbed in the semiconductorfilm. In the case of using a silicon film as the semiconductor film, thelaser beam emitted from the laser oscillator preferably has a wavelengthof 600 nm or shorter in consideration of the absorption coefficient. Asthe laser oscillator emitting such a wavelength, there are an excimerlaser, a YAG laser (harmonic), and a glass laser (harmonic), forexample.

In addition, although high output power is not yet obtained by thecurrent technology. a YVO₄ laser (harmonic), a YLF laser (harmonic), anAr laser, and an GdVO₄ are given for example as the laser oscillatoremitting the laser beam having a wavelength appropriate forcrystallizing the silicon film.

The optical system disclosed in the present invention may be used underthe atmosphere or may be used under a nitrogen or Ar atmosphere in orderto minimize the damage of the light pipe and the surface of the lens dueto the laser beam having high energy.

Although this embodiment mode explains the optical element having a pairof reflection planes provided oppositely to homogenize the energydistribution of the beam spot, the light pipe or the optical waveguidemay be also used because they have the similar optical advantageouseffect.

Hereinafter a method for manufacturing a semiconductor device using thebeam homogenizer and the laser irradiation apparatus of the presentinvention is explained. A glass substrate having enough resistanceagainst the heat up to 600 r is used as the substrate. A silicon oxidefilm is formed on the glass substrate as a base film, and a non-singlecrystal silicon film is formed thereon. These films are formed by asputtering method or a plasma CVD method.

The substrate with the films formed thereon is heated under the nitrogenatmosphere to decrease the concentration of hydrogen in the non-singlecrystal silicon film. This process is performed because the film cannotresist the laser power when the film contains too much hydrogen. Theconcentration of hydrogen in the film is appropriate on the order of10²⁰ atoms/cm³. Here, 10²⁰ atoms/cm³ means that 10²⁰ hydrogen atomsexist in 1 cm³. The processing time and the temperature of the substratein this heating process may be determined by a practitionerappropriately. However, the heating temperature must be determined inconsideration of the resistivity of the glass substrate. This embodimentmode employs a XeCl excimer laser as the laser oscillator.

The excimer laser is a pulsed laser oscillator. When the power of thepulsed laser beam fluctuates within ±5%, preferably within ±2%, in eachpulse during the laser processing performed to one substrate, it ispossible to perform homogeneous crystallization. It is noted that thelenses and the optical element that has a curved shape in its entrancesurface for homogenizing the energy distribution of the laser beam shownin the present embodiment mode are made of the synthetic quartz havinghigh transmittance and resistance to the XeCl excimer laser.

The fluctuation of the laser power described above is defined asfollows. The average value of the laser power in the period of theirradiation to one substrate is assumed to be standard. Then, thefluctuation of the laser power is defined as the value expressing thedifference between the average value and the maximum or minimum value inthe period of the irradiation in percentage terms.

The laser beam is irradiated in such a way that a stage with theirradiated surface 307 shown in FIGS. 3A and 3B mounted thereon isscanned in the direction of the short side of the rectangular beam spot.On this occasion, a practitioner may determine the energy density andthe scanning speed of the beam spot on the irradiated surfaceappropriately. The energy density is appropriate in the range of 200 to1000 mJ/cm². It is feasible to perform laser annealing homogeneouslywhen the scanning speed is selected in the range where the width of therectangular beam spot in its short side is overlapped one another by 80%or more, preferably by approximately 90%. The optimum scanning speeddepends on the pulse repetition rate of the laser oscillator and it maybe regarded to be proportional to the pulse repetition rate thereof.

Thus, the laser annealing process is completed. When the above step isperformed repeatedly, many substrates can be processed. The substrateprocessed thus can be utilized to manufacture an active matrix liquidcrystal display and an EL display device according to the known method.

The above example used the excimer laser as the laser oscillator. Sincethe excimer laser has a coherent length as short as several μm, it issuitable for the above optical system. Although some of the lasers shownbelow have the long coherent length, the laser whose coherent length ischanged deliberately may be employed. It is also preferable to use theharmonic of the YAG laser or the harmonic of the glass laser because theenergy of the laser beam is sufficiently absorbed in the silicon film.As the laser oscillator appropriate for the crystallization of thesilicon film, the YVO₄ laser (harmonic), the YLF laser (harmonic), theAr laser, the GdVO₄ laser, and the like are given. The wavelengths ofthese laser beams are sufficiently absorbed in the silicon film.

The above example used the non-single crystal silicon film as thenon-single crystal semiconductor film. However, it is easily supposedthat the present invention can be applied to another non-single crystalsemiconductor film. For example, a compound semiconductor film such as anon-single crystal silicon germanium film or a poly-crystalline silicongermanium film may be employed as the non-single crystal semiconductorfilm. Alternatively, a poly-crystalline silicon film may be used as thenon-single crystal semiconductor film. For example, the poly-crystallinesilicon film can be formed as follows.

A silicon oxide film is formed on a glass substrate and a nonsingle-crystal silicon film is further formed thereon. The silicon oxidefilm and the non-single crystal silicon film are formed by thesputtering method or the CVD method. In addition, plasma CVD may beused. After that, a silicon oxide film may be formed on the non-singlecrystal silicon film by applying a hydroxy solution to the non-singlecrystal silicon film. Forming this silicon oxide film is because thefollowing process of applying an acetate solution including nickel canbe performed so that the acetate solution can be spread all over thesurface of the non-single crystal silicon film. For example, when theacetate solution is applied directly on the surface of the non-singlecrystal silicon film, the acetate solution is repelled on the non-singlecrystal silicon film, and therefore the nickel cannot be applieduniformly all over the surface of the non-single crystal silicon film.Therefore, the wettability can be improved by forming the silicon oxidefilm. Next, an acetate solution including nickel in the range of 1 to100 ppm is applied to the non-single crystal silicon film. After that,the heating process is performed to crystallize the non-single crystalsilicon film and to form a crystalline silicon film.

Embodiment 1

An example of an optical system used in the present embodiment isexplained with reference to FIGS. 4A and 4B. A side view of FIG. 4B isexplained first. A laser beam emitted from a XeCl excimer laseroscillator 401 propagates in a direction indicated by an arrow in FIGS.4A and 4B. First, the laser beam is expanded by spherical lenses 402 aand 402 b. The spherical lenses 402 a and 402 b are not necessary whenthe beam spot emitted from the laser oscillator 401 is sufficientlylarge. It is noted that the lenses and the light pipe that has a curvedshape in its entrance surface for homogenizing the energy distributionof the laser beam shown in the present embodiment are made of syntheticquartz having high transmittance and resistance to the XeCl excimerlaser.

The direction of the long side herein described mean the direction ofthe long side of the rectangular beam spot formed on an irradiatedsurface 408. The direction of the short side herein described mean thedirection of the short side of the rectangular beam spot formed on theirradiated surface 408. A cylindrical lens 405 has a first surfacehaving a radius of curvature of 486 mm, a second surface that is plain,and a thickness of 20 mm, which focuses the beam spot in a direction ofthe short side. It is noted that the first surface of the lens is thesurface into which the light is incident, and the second surface meansthe surface from which the light is emitted. The sign of the radius ofcurvature is positive when the center of the curvature is in the sidewhere the beam is emitted with respect to the lens surface. The sign isnegative when the center of the curvature is in the side where the beamis incident with respect to the lens surface. A light pipe 406positioned 1000 mm behind the cylindrical lens 405 has a pair ofreflection planes provided oppositely for reflecting the laser beam inthe direction where the energy distribution of the beam spot ishomogenized and a cylindrical shape in the entrance surface thereofhaving a radius of curvature of −38 mm. The laser beam incident into thelight pipe 406 is totally reflected in the light pipe 406 repeatedly andis led to the exit. This homogenizes the energy distribution of therectangular beam spot in the direction of its short side on theirradiated surface. The direction of the cylindrical curvature is thedirection of the short side, which is the direction where the energydistribution is homogenized. The light pipe 406 has a length of 250 mmin a direction where the laser beam is incident and has a distance of 2mm between the total-reflection planes.

In the present embodiment, the light pipe 406 is made of the syntheticquartz having a refractive index of approximately 1.486 to a wavelengthof 308 nm, which is higher than that of the air (the air has arefractive index of approximately 1). Since the laser beam is incidentinto the light pipe 406 at a critical angle or more in this embodiment,the laser beam is totally reflected at the reflection plane. In otherwords, the transmittance of the light in the light pipe is high comparedwith the case where the laser beam is not totally reflected. Therefore,the laser beam emitted from the laser oscillator 401, which is the lightsource, can be focused at the irradiated surface 408 more efficiently.

A cylindrical lens 407 positioned 1250 mm behind the light pipe 406 hasa first surface having a radius of curvature of 97 mm, a second surfacethat is plane, and a thickness of 30 mm. The cylindrical lens 407focuses the laser beam on the irradiated surface 408 positioned 220 mmbehind the cylindrical lens 407 in a direction of the short side of therectangular beam spot. In other words, the cylindrical lens 407 projectsthe plane having homogeneous energy distribution formed at the exitsurface of the light pipe 406 to the irradiated surface 408. Thishomogenizes the energy distribution of the rectangular beam spot in thedirection of its short side and determines the length thereof in thedirection of its short side.

Next, a top view of FIG. 4A is explained. The beam spot of the laserbeam emitted from the laser oscillator 401 is divided by a cylindricallens array 403 in a direction of the long side of the rectangular beamspot. The cylindrical lens array 403 has seven cylindrical lensesarranged in a direction of its curvature, each of which has a firstsurface having a radius of curvature of 24.5 mm, a second surface thatis plane, a width of 6.5 mm in a direction of its long side, and athickness of 5 mm.

A cylindrical lens 404 positioned 500 mm behind the cylindrical lensarray 403 has a first surface having a radius of curvature of 2140 mmand a second surface that is plane. The cylindrical lens 404 combinesthe laser beams divided by the cylindrical lens array 403 on theirradiated surface 408. This homogenizes the energy distribution of therectangular beam spot in the direction of its long side and determinesthe length thereof in the direction of its long side. It is noted thatthe cylindrical lens 404 is not used in the embodiment mode of thepresent invention. The cylindrical lens 404 can decrease the portionwhere the energy attenuates in opposite ends of the rectangular beamspot in a direction of its long side. However, when this lens is used inthe apparatus having the above structure, the lens may have an extremelylong focal length. In such a case, since the advantageous effect by thislens decreases, it may not be used.

As described above, even though the beam axis changes, it is possible toform a rectangular beam spot having homogeneous energy distribution andhaving a size of 320 mm in the long side and 0.4 mm in the short side byusing the beam homogenizer including the optical element having thecurved shape in the entrance surface thereof.

The optical system shown in the present embodiment is used to performthe laser annealing to the semiconductor film according to the methodshown in the embodiment mode of the present invention for example.Moreover, the semiconductor film annealed thus can be used tomanufacture an active matrix liquid crystal display or an EL displaydevice. A practitioner may manufacture these by the known method.

Embodiment 2

The present embodiment explains an example of an optical systemdifferent from that described above. FIGS. 6A and 6B show the example ofthe optical system explained in this embodiment. In addition, the lensesshown in the present embodiment are made of synthetic quartz having hightransmittance and resistance to the XeCl excimer laser.

In FIGS. 6A and 6B, the laser beam travels in the same optical path asthat shown in FIGS. 4A and 4B in the embodiment 1 except for an opticalelement 606 that has a curved shape in its entrance surface forhomogenizing the energy distribution of the laser beam. The opticalelement 606 has a pair of reflection planes provided oppositely as wellas the light pipe 406. The light pipe 406 in FIGS. 4A and 4B hascurvature in the entrance surface thereof and is made of syntheticquartz having a refractive index of approximately 1.486 to the XeClexcimer laser. The laser beam incident into the light pipe 406 istotally reflected in the light pipe repeatedly and is led to the exit.On the other hand, the optical element 606 has a pair of mirrors 606 aand 606 b provided oppositely and has a cylindrical lens 607 positionedin the entrance of the optical element 606. The space between the pairof mirrors 606 a and 606 b is a hollow space except for the cylindricallens 607. The light pipe 406 and the optical element 606 are differentin this point. The optical element 606 has a length of 250 mm in adirection of the beam axis and the distance between the pair of mirrorsis 2 mm. The cylindrical lens 607 has a first surface having a radius ofcurvature of −38 mm, a second surface that is plane, and a thickness of5 mm. The laser beam incident into the optical element 606 is expandedin a direction of the short side of the rectangular beam spot by thecylindrical lens 607, and the energy distribution is homogenized whilethe laser beam is reflected in the optical element 606 symmetrically tothe center axis of the optical element 606.

The optical system shown in the present embodiment is simulated and therectangular beam spot is observed. FIG. 5A shows the result of theoptical simulation of the optical system in which the incidence angle ofthe laser beam into the light pipe 406 is set to 0°. The incidence angleherein described is defined as follows with reference to FIG. 16. It isnoted that FIG. 16 is an enlarged view of the cylindrical lens 405 andthe optical element 606. The incidence angle is an angle θ between thecenter axis of the optical element 606 shown by a dot-dashed line inFIG. 16 and a dotted line in FIG. 16 connecting the vertex of thecylindrical lens 405 and an intersection of the center axis of theoptical element 606 with the entrance surface thereof in the planeparallel to the center axis of the optical element 606 including thedirection of the short side of the rectangular beam spot. The energydistribution in the direction of the short side can be made homogeneousas shown in FIG. 5A. According to another optical simulation of theoptical system in which the incidence angle θ of the laser beam into theoptical element 606 is set to 0.086°, homogeneous energy distribution isobtained as shown in FIG. 5B. On the other hand, according to theoptical simulation of the optical system in which the incidence angle θof the laser beam into the optical element 606 is set to 0.086° when theentrance surface of the optical element 606 is plane, inhomogeneousenergy distribution is obtained as shown in FIG. 5C.

As described above, even though the beam axis changes, it is possible toform a rectangular beam spot having homogeneous energy distribution andhaving a size of 320 mm in the long side and 0.4 mm in the short side byusing the beam homogenizer including the optical element having thecurved shape in the entrance surface thereof.

In addition, FIGS. 13A to 15B show the result of another opticalsimulation. Specifically, they show the energy distribution in adirection of the short side. In this optical simulation, the opticalsystem is set so that the laser beam is incident into the opticalelement 606 at an incidence angle of 0.17° and the entrance surface ofthe optical element 606 has various radiuses of curvature. The radiusesof curvature of the optical element 606 in FIGS. 13A to 15B are −300 mm,−100 mm, −50 mm, −38 mm, −26 mm, and −20 mm respectively. The energydistribution is inhomogeneous in FIGS. 13A and 13B, while the energydistribution is homogeneous in FIGS. 14A to 15B in which the radius ofcurvature ranges from −50 to −20 mm. However, the laser beam emittedfrom the optical element having a radius of curvature of −20 mm in FIG.15B is expanded so as to be larger than the size of the cylindrical lens407, and only the ray incident into the cylindrical lens 407 ishomogenized. In order to homogenize all the rays emitted from theoptical element, the size of the cylindrical lens 407 is enlarged or thedistance between the cylindrical lens 407 and the optical element 606 isshortened.

As above, with the optical element having a radius of curvature of −50mm or less, it is possible to homogenize the energy distribution of thelaser beam on the irradiated surface even though the beam axis changes.

The optical system shown in the present embodiment is used to performthe laser annealing to the semiconductor film according to the methodshown in the embodiment mode of the present invention for example.Moreover, the semiconductor film annealed thus can be used tomanufacture an active matrix liquid crystal display or an EL displaydevice. A practitioner may, manufacture these by the known method.

Embodiment 3

This embodiment explains an example of an optical system different fromthat described above. FIGS. 7A and 7B show the example of the opticalsystem explained in this embodiment. It is noted that the lenses shownin the present embodiment are made of synthetic quartz having hightransmittance and resistance to the XeCl excimer laser.

In FIGS. 7A and 7B, the laser beam travels in the same optical path asthat shown in FIGS. 6A and 6B in the embodiment 2 except for acylindrical lens 707. An optical element 706 has a pair of reflectionplanes provided oppositely as well as the optical element 606. In FIGS.6A and 6B, the laser beam incident into the optical element 606 isexpanded in a direction of the short side of the rectangular beam spotby a concave cylindrical lens 607 provided in the vicinity of theentrance of the optical element 606. On the other hand, in FIGS. 7A and7B, the laser beam incident into the optical element 706 is focused by aconvex cylindrical lens 707 provided in the vicinity of the entrance ofthe optical element 706 in the direction of the short side of therectangular beam spot and is expanded thereafter. The optical elements607 and 707 are different in this point. The optical element 706 has alength of 250 mm in the direction of the beam axis and the distancebetween the pair of mirrors is 2 mm. The cylindrical lens 707 has afirst surface having a radius of curvature of 38 mm, a second surfacethat is plane, and a thickness of 5 mm. The laser beam incident into theoptical element 706 is focused and then expanded in the direction of theshort side of the rectangular beam spot by the cylindrical lens 707, andthe energy distribution thereof is homogenized while the laser beam isreflected in the optical element 706 symmetrical to the center axis ofthe optical element 706.

The optical system shown in FIGS. 7A and 7B forms a rectangular beamspot having homogeneous energy distribution and having a size of 0.4 mmin the short side and 320 mm in the long side on the irradiated surface709.

The optical system shown in the present embodiment is used to performthe laser annealing to the semiconductor film according to the methodshown in the embodiment mode of the present invention for example.Moreover, the semiconductor film annealed thus can be used tomanufacture an active matrix liquid crystal display or an EL displaydevice. A practitioner may manufacture these by the known method.

Embodiment 4

This embodiment explains an example of an optical system including alight pipe for homogenizing the energy distribution of the rectangularbeam spot in the direction of its long side. FIGS. 8A and 8B show theexample of the optical system explained in this embodiment. It is notedthat the lenses and the light pipe that has a curved shape in itsentrance surface for homogenizing the energy distribution of the laserbeam shown in the present embodiment are made of synthetic quartz havinghigh transmittance and resistance to the XeCl excimer laser. It is notedthat the direction of the long side herein referred to means thedirection of the long side of the rectangular beam spot formed on anirradiated surface 808, and the direction of the short side means thedirection of the short side of the rectangular beam spot formed on theirradiated surface 808.

First, a top view of FIG. 8A is explained. A laser beam emitted from aXeCl excimer laser oscillator 801 propagates in a direction indicated byan arrow in FIGS. 8A and 8B. A cylindrical lens 802 has a first surfacehaving a radius of curvature of 194.25 mm, a second surface that isplane, and a thickness of 20 mm, which focuses the beam spot in thedirection of the long side. An entrance surface of a light pipe 803positioned 400 mm behind the cylindrical lens 802 has a cylindricalshape having a radius of curvature of −50 mm, the light pipe 803 havinga pair of reflection planes provided oppositely for reflecting the laserbeam in the direction where the energy distribution of the beam spot ishomogenized. The laser beam incident into the light pipe 803 is totallyreflected in the light pipe 803 repeatedly and is led to the exit. Thishomogenizes the energy distribution of the rectangular beam spot in thedirection of the long side on the irradiated surface. It is noted thatthe direction of curvature is the direction of the long side, whichmeans the direction where the energy distribution is homogenized Thelight pipe 803 has a length of 300 mm in a direction of the beam axisand the distance between the total-reflection planes is 2 mm.

A cylindrical lens 804 positioned 20 mm behind the light pipe 803 has afirst surface having a radius of curvature of 9.7 mm, a second surfacethat is plane, and a thickness of 5 mm. The cylindrical lens 804 focusesthe laser beam emitted from the light pipe 803 on an irradiated surface808 positioned 3600 mm behind the cylindrical lens 804. In other words,the plane having homogeneous energy distribution formed at the exitsurface of the light pipe 803 is projected to the irradiated surface 808by the cylindrical lens 804. This homogenizes the energy distribution ofthe rectangular beam spot in the direction of the long side anddetermines the length thereof in the direction of the long side.

Next, a side view of FIG. 8B is explained. A beam spot of a laser beamemitted from a laser oscillator 801 is divided by cylindrical lensarrays 805 a and 805 b in the direction of the short side. Thecylindrical lens array 805 a has seven cylindrical lenses arranged inthe direction of its curvature, each of which has a first surface havinga radius of curvature of 200 mm, a second surface that is plane, athickness of 5 mm, and a width of 4 mm in a direction of the short side.The cylindrical lens array 805 b has seven cylindrical lenses arrangedin the direction of its curvature, each of which has a first surfacethat is plane, a second surface having a radius of curvature of 160 mm,a thickness of 5 mm, and a width of 4 mm in a direction of the shortside. The beam spots divided by the cylindrical lens arrays 805 a and805 b are combined and focused by a cylindrical lens 806 having a firstsurface with a radius of curvature of 486 mm, a second surface that isplane, and a thickness of 20 mm. Thus, a plane having homogeneous energydistribution and having a length of 2 mm in the short side is formed inthe position 1000 mm behind the cylindrical lens 806.

Moreover, a doublet cylindrical lens 807 positioned 1250 mm behind thecylindrical lens 806 projects the plane having homogeneous energydistribution to the irradiated surface 808 positioned 230 mm behind thedoublet cylindrical lens 807. This homogenizes the energy distributionof the rectangular beam spot in the direction of the short side anddetermines the length thereof in the direction of the short side. Thedoublet cylindrical lens 807 consists of a cylindrical lens 807 a and acylindrical lens 807 b. The cylindrical lens 807 a has a first surfacehaving a radius of curvature of 125 mm, a second surface having a radiusof curvature of 77 mm, and a thickness of 10 mm. The cylindrical lens807 b has a first surface having a radius of curvature of 97 mm, asecond surface having a radius of curvature of −200 mm, and a thicknessof 20 mm. The distance between the cylindrical lenses 807 a and 807 b is5.5 mm.

The optical system shown in FIGS. 8A and 8B forms a rectangular beamspot having homogeneous energy distribution and having a size of 0.4 mmin the short side and 320 mm in the long side on the irradiated surface808. FIG. 9 shows the energy distribution of the rectangular beam spotin the direction of the long side formed by the optical system shown inFIGS. 8A and 8B.

The optical system shown in the present embodiment is used to performthe laser annealing to the semiconductor film according to the methodshown in the embodiment mode of the present invention for example.Moreover, the semiconductor film annealed thus can be used tomanufacture an active matrix liquid crystal display or an EL displaydevice. A practitioner may manufacture these by the known method.

Embodiment 5

This embodiment explains an example of an optical system different fromthose shown in the above embodiments. FIGS. 17A and 17B show the exampleof the optical system explained in this embodiment. It is noted that thelenses shown in the present embodiment are made of synthetic quartzhaving high transmittance and resistance to the XeCl excimer laser.

In FIGS. 17A and 17B, the laser beam travels along the same optical pathas that shown in the embodiment 4 with reference to FIGS. 8A and 8Bexcept for an optical element 1503 having a curved shape in its entrancesurface for homogenizing the energy distribution of the laser beam. Theoptical element 1503 has a pair of reflection planes provided oppositelyas well as the light pipe 803. In FIGS. 4A and 4B, the laser beampropagates in the light pipe 406 made of the synthetic quartz having arefraction index of approximately 1.486 to the XeCl excimer laser thathas a curved shape in its entrance surface while repeatingtotal-reflection and is led to the exit. On the other hand, in FIGS. 8Aand 8B, the optical element 1503 consists of a pair of mirrors 1503 aand 1503 b provided oppositely and a cylindrical lens 1504 providedtherebetween where the laser beam is incident. The space between thepair of mirrors is filled with air except for the cylindrical lens 1504.The optical element 1503 and the light pipe 406 are different in thispoint. The distance between the pair of mirrors 1503 a and 1503 b is 2mm and the length of the optical element 1503 is 300 mm in a directionof the beam axis. The cylindrical lens 1504 has a first surface having aradius of curvature of −50mm, a second surface that is plane, and athickness of 5 mm. The energy distribution of the laser beam ishomogenized in such a way that the laser beam incident into the opticalelement 1503 is expanded in a direction of the long side of therectangular beam spot by the cylindrical lens 1504 and is led to theexit while reflecting in the optical element 1503 symmetrical to thecenter axis of the optical element 1503.

The optical system shown in FIGS. 17A and 17B forms the rectangular beamspot having homogeneous energy distribution and having a size of 320 mmin the long side and 0.4 mm in the short side on the irradiated surface808. FIG. 9 shows the energy distribution of the rectangular beam spotin the direction of its long side formed by the optical system shown inFIGS. 17A and 17B.

The optical system shown in the present embodiment is used to performthe laser annealing to the semiconductor film according to the methodshown in the embodiment mode of the present invention, for example.Moreover, the semiconductor film annealed thus can be used tomanufacture an active matrix liquid crystal display or an EL displaydevice. A practitioner may manufacture these by the known method.

Embodiment 6

The present embodiment explains an example where the light pipe is usedto homogenize the energy distribution of a rectangular beam spot indirections of its long and short direction. FIGS. 10A and 10B show theexample of the optical system explained in this embodiment. The lensesand the light pipe having a curved shape in its entrance surface forhomogenizing the energy distribution of the laser beam are made ofsynthetic quartz having high transmittance and high resistance to theXeCl excimer laser. Hereinafter, the direction of the long side meansthe direction of the long side of the rectangular beam spot formed onthe irradiated surface 1008, and the direction of the short side meansthe direction of the short side of the rectangular beam spot formed onthe irradiated surface 1008.

First, a top view of FIG. 10A is explained. A laser beam emitted from aXeCl excimer laser oscillator 1001 propagates in a direction indicatedby an arrow in FIGS. 10A and 10B. A cylindrical lens 1002 has a firstsurface having a radius of curvature of 194.25 mm, a second surface thatis plane, and a thickness of 20 mm, which focuses the beam spot in thedirection of the long side. An entrance surface of a light pipe 1003positioned 400 mm behind the cylindrical lens 1002 has a cylindricalshape having a radius of curvature of −38 mm, the light pipe 1003 havinga pair of reflection planes provided oppositely for reflecting the laserbeam in the direction where the energy distribution of the beam spot ishomogenized. The laser beam incident into the light pipe 1003 is totallyreflected in the light pipe 1003 repeatedly and is led to the exit. Thishomogenizes the energy distribution of the rectangular beam spot in thedirection of the long side on the irradiated surface. It is noted thatthe direction of curvature is the direction of the long side, whichmeans the direction where the energy distribution is homogenized. Thelight pipe 1003 has a length of 300 mm in the direction of the beam axisand the distance between the total-reflection planes is 2 mm.

A cylindrical lens 1004 positioned 20 mm behind the light pipe 1003 hasa first surface having a radius of curvature of 9.7 mm, a second surfacethat is plane, and a thickness of 5 mm. The cylindrical lens 1004focuses the laser beam emitted from the light pipe 1003 on an irradiatedsurface 1008 positioned 3600 mm behind the cylindrical lens 1004. Inother words, the plane having homogeneous energy distribution formed atthe exit surface of the light pipe 1003 is projected to the irradiatedsurface 1008 by the cylindrical lens 1004. This homogenizes the energydistribution of the rectangular beam spot in the direction of the longside and determines the length thereof in the direction of the longside.

Then, a side view of FIG. 10B is explained. The beam spot of the laserbeam emitted from the XeCl excimer laser oscillator 1001 is focused inthe direction of the short side by a cylindrical lens 1005 having afirst surface with a radius of curvature of 486 mm, a second surfacethat is plane, and a thickness of 20 mm. An entrance surface of a lightpipe 1006 positioned 1000 mm behind the cylindrical lens 1005 has acylindrical shape having a radius of curvature of −38 mm, the light pipe1006 having a pair of reflection planes provided oppositely forreflecting the laser beam in the direction where the energy distributionof the beam spot is homogenized. The laser beam incident into the lightpipe 1006 is totally reflected in the light pipe 1006 and is led to theexit. This homogenizes the energy distribution of the rectangular beamspot in the direction of the short side on the irradiated surface. It isnoted that the direction of curvature is the direction of the shortside, which means the direction where the energy distribution ishomogenized. The light pipe 1006 has a length of 250 mm in the directionof the beam axis and the distance between the total-reflection planes is2 mm.

In the present embodiment, the light pipes 1003 and 1006 are made ofsynthetic quartz having a refractive index of approximately 1.486 to thewavelength of 308 nm, which is higher than the refractive index of theair (the air has a refractive index of approximately 1). Since the laserbeam is incident into the light pipes 1003 and 1006 at a critical angleor more in the present embodiment, the laser beam is totally reflectedon the reflection planes. In other words, the transmittance of the laserbeam in the light pipe is high compared to the case in which the laserbeam is not totally reflected. Therefore, the laser beam emitted fromthe laser oscillator 1001, which is the light source, can be focused onthe irradiated surface 1008 more efficiently.

A cylindrical lens 1007 positioned 1250 mm behind the light pipe 1006has a first surface having a radius of curvature of 97 mm, a secondsurface that is plane, and a thickness of 30 mm. The cylindrical lens1007 focuses the laser beam emitted from the light pipe 1006 on theirradiated surface 1008 positioned 200 mm behind the cylindrical lens1007. In other words, the plane having homogeneous energy distributionformed at the exit surface of the light pipe 1006 is projected to theirradiated surface 1008 by the cylindrical lens 1007. This homogenizesthe energy distribution of the rectangular beam spot in the direction ofthe short side and determines the length thereof in the direction of theshort side.

The above optical system forms the rectangular beam spot havinghomogeneous energy distribution and having a size of 320 mm in the longside and 0.4 mm in the short side.

The optical system shown in the present embodiment is used to performthe laser annealing to the semiconductor film according to the methodshown in the embodiment mode of the present invention for example.Moreover, the semiconductor film annealed thus can be used tomanufacture an active matrix liquid crystal display or an EL displaydevice. A practitioner may manufacture these by the known method.

1. A method for manufacturing a semiconductor device comprising: forminga semiconductor film over a substrate, and irradiating the semiconductorfilm with a laser beam while moving a position of a beam spot relativeto the substrate, wherein the laser beam is emitted from a laseroscillator and is transmitted through an optical system so as to changea sectional shape of the laser beam, wherein the optical system includesan optical element for homogenizing energy distribution of the laserbeam, wherein the optical element has a pair of reflection planesprovided oppositely for reflecting the laser beam in a direction wherethe energy distribution is homogenized, and wherein the laser beamenters a curved surface of the optical element.
 2. The method formanufacturing a semiconductor device according to claim 1, wherein theoptical element is a light pipe.
 3. The method for manufacturing asemiconductor device according to claim 1, wherein the optical elementis an optical waveguide.
 4. The method for manufacturing a semiconductordevice according to claim 1, wherein the curved surface has curvature ina direction where the optical element acts.
 5. The method formanufacturing a semiconductor device according to claim 1, wherein thebeam spot has an aspect ratio of 10 or more.
 6. The method formanufacturing a semiconductor device according to claim 1, wherein thebeam spot has an aspect ratio of 100 or more.
 7. The method formanufacturing a semiconductor device according to claim 1, wherein thelaser oscillator is one selected from the group consisting of an excimerlaser, a YAG laser, a glass laser, a YVO₄ laser, a YLF laser, an Arlaser, and a GdVO₄ laser.
 8. A method for manufacturing a semiconductordevice comprising: forming a semiconductor film over a substrate, andirradiating the semiconductor film with a laser beam while moving aposition of a beam spot relative to the substrate, wherein the laserbeam is emitted from a laser oscillator and is transmitted through anoptical system so as to change a sectional shape of the laser beam,wherein the optical system includes an optical element for homogenizingenergy distribution of the laser beam and one or a plurality ofcylindrical lenses for projecting a plane having homogeneous energydistribution formed by the optical element to the semiconductor film,wherein the optical element has a pair of reflection planes providedoppositely for reflecting the laser beam in a direction where the energydistribution is homogenized, and wherein the laser beam enters a curvedsurface of the optical element.
 9. The method for manufacturing asemiconductor device according to claim 8, wherein the optical elementis a light pipe.
 10. The method for manufacturing a semiconductor deviceaccording to claim 8, wherein the optical element is an opticalwaveguide.
 11. The method for manufacturing a semiconductor deviceaccording to claim 8, wherein the curved surface has curvature in adirection where the optical element acts.
 12. The method formanufacturing a semiconductor device according to claim 8, wherein thebeam spot has an aspect ratio of 10 or more.
 13. The method formanufacturing a semiconductor device according to claim 8, wherein thebeam spot has an aspect ratio of 100 or more.
 14. The method formanufacturing a semiconductor device according to claim 8, wherein thelaser oscillator is one selected from the group consisting of an excimerlaser, a YAG laser, a glass laser, a YVO₄ laser, a YLF laser, an Arlaser, and a GdVO₄ laser.
 15. A method for manufacturing a semiconductordevice comprising: forming a semiconductor film over a substrate, andirradiating the semiconductor film with a laser beam while moving aposition of a beam spot relative to the substrate, wherein the laserbeam is emitted from a laser oscillator and is transmitted through anoptical system so as to change a sectional shape of the laser beam,wherein the optical system includes an optical element for homogenizingenergy distribution of the laser beam, wherein the optical element has apair of reflection mirrors provided oppositely for reflecting the laserbeam in a direction where the energy distribution is homogenized,wherein the laser beam enters a cylindrical lens between the reflectionmirrors.
 16. The method for manufacturing a semiconductor deviceaccording to claim 15, wherein the optical element is a light pipe. 17.The method for manufacturing a semiconductor device according to claim15, wherein the optical element is an optical waveguide.
 18. The methodfor manufacturing a semiconductor device according to claim 15, whereinthe cylindrical lens has curvature in a direction where the opticalelement acts.
 19. The method for manufacturing a semiconductor deviceaccording to claim 15, wherein the beam spot has an aspect ratio of 10or more.
 20. The method for manufacturing a semiconductor deviceaccording to claim 15, wherein the beam spot has an aspect ratio of 100or more.
 21. The method for manufacturing a semiconductor deviceaccording to claim 15, wherein the laser oscillator is one selected fromthe group consisting of an excimer laser, a YAG laser, a glass laser, aYVO₄ laser, a YLF laser, an Ar laser, and a GdVO₄ laser.